First of all, the architecture of a known polymer electrolyte fuel cell (hereinafter referred to as “PEFC” according to need) will be outlined.
PEFC is a cogeneration system that generates electric power and heat at the same time through an electro-chemical reaction between a fuel gas containing hydrogen and an oxidizing gas (e.g., air) containing oxygen, in a fuel cell.
This fuel cell includes a membrane electrode assembly abbreviated to “MEA”. In one example of MEA constructions, a catalyst reaction layer (gas diffusion electrode) containing, as a chief component, a carbon powder that carries a metal (e.g., platinum) having catalytic ability is arranged on both surfaces of a polymer electrolyte membrane that selectively transports hydrogen ions. A gas diffusion layer (gas diffusion electrode) having permeability to the fuel gas and oxidizing gas (i.e., reaction gases for power generation) as well as electron conductivity is arranged on the outer side of each catalyst reaction layer.
A gasket (gas sealing member) for preventing gas leak and gas commixture is provided on the periphery of each surface of the MEA. This MEA is held between a pair of electrically-conductive separator plates, that is, an anode separator plate and a cathode separator plate.
Such an MEA, gas sealing members and separator plates are piled so that about 10 to 200 units of MEA each sandwiched between the pair of electrically-conductive separator plates are stacked. Then, these members are held between end plates with a current collecting plate and an insulating plate interposed therebetween and secured by fastening bolts inserted from both ends.
Each separator plate is provided with gas inlet manifold holes and gas outlet manifold holes. Each gas inlet manifold hole allows passage of its associated reaction gas supplied to the fuel cell. Each gas outlet manifold hole allows passage of a gas/liquid mixture fluid including the reaction gas which has been left after the power generation of the fuel cell and water generated through an electro-chemical reaction at the electrode.
Formed on the surface of each separator plate in contact with the MEA are a plurality of gas passage grooves functioning to guide the reaction gas to the gas diffusion electrode of the MEA and send the gas/liquid mixture fluid of generated water and off gas. The gas passage grooves are formed so as to wind, in so-called serpentine form, between their associated gas inlet manifold hole and gas outlet manifold hole.
Although the gas passages for flowing the reaction gas can be formed separately of the separator plate, it is common practice to form a concavo-convex pattern, constituted by a plurality of concave portions (grooves) and a plurality of convex portions (ribs), on the surface of the separator plate as described earlier and to use the concave grooves of this pattern as the gas passages.
The formation of a plurality of such serpentine-shaped gas passage grooves is desirable because it can reduce the resistance of the gas passages laid between the gas inlet manifold hole and the gas outlet manifold hole. Separator plates provided with gas passage grooves of serpentine shape have already been disclosed in many documents (see e.g., FIG. 11 of Patent Document 1 and FIG. 4 of Patent Document 3).
Each of the gas passage grooves provided in each separator plate is a closed space that is defined by the concave portion of the concavo-convex pattern formed on the surface of the separator plate and a surface of the gas diffusion electrode serving as a porous film of the MEA and that has a substantially rectangular sectional shape.
Since it is inevitable that the adjacent serpentine-shaped gas passage grooves are different in flow path length in some regions, a pressure difference appears in these regions, causing “a gas movement (transmission) by way of the gas diffusion electrode” between the adjacent gas passage grooves.
If the degree of such a gas movement between the gas passage grooves exceeds a certain level, uniform feeding of the reaction gas to the MEA may be interrupted by the variation in the gas flow rate between the gas passage grooves. In addition, the gas passage grooves, which lack in the flow volume of gas owing to the variation in the gas flow rate between the gas passage grooves, will be subjected to a significant increase in passage resistance because of a decrease in the ability of discharging the water generated through the electro-chemical reaction of the fuel cell and the vapor flocculated water contained in the reaction gas (these waters are hereinafter referred to as “condensed water”). This escalates the gas movement and, finally, a voltage drop phenomenon may occur owing to the lack of the reaction gas caused by flooding.
It should be noted that the “flooding” as stated hereinabove is a phenomenon that appears in the gas passage grooves of the separator plates when clogged with water droplets and is different from the phenomenon (i.e., flooding within the gas diffusion electrodes) in the gas diffusion electrodes (e.g., the pores serving as gas diffusion paths in the catalyst layers) clogged with water droplets.
As attempts to properly suppress variations in the flow rate and pressure of the reaction gas between a plurality of serpentine-shaped gas passage grooves formed in a separator plate, there have been proposed various techniques such as proper segmentation of the serpentine-shaped gas passage grooves.
One example of such techniques is a separator plate provided with a grid-like projection pattern. This pattern is defined, in grid-like form, in the turn portions of the plurality of gas passage grooves with the intent of uniformizing the flow rate and pressure of the reaction gas between the gas passage grooves (see the prior art disclosed in Patent Document 2).
Another example of the techniques is a separator plate in which the plurality of gas passage grooves are connected by communicating grooves to thereby uniformize the flow rate and pressure of the reaction gas between the plurality of gas passage grooves (see the prior art disclosed in Patent Document 3).
Patent Document 1: JP-A-2000-100458
Patent Document 2: JP-A-2000-164230
Patent Document 3: JP-A-2004-220950